Leaf anatomy, photosynthesis and chlorophyll fluorescence of lettuce as influenced by arbuscular mycorrhizal fungi under high temperature stress

Abstract

In order to elucidate the mechanism of how arbuscular mycorrhiza (AM) fungi offer protection in lettuce (Lactuca sativa L.) from high temperature stress, the effects of mycorrhizal fungi, Funneliformis mosseae, on chloroplast ultrastructure, photosynthetic pigments, chlorophyll fluorescence of lettuce (cv. Shuangzi) grown under temperature at 25℃ and 35℃, respectively, were investigated in this study. Results showed that non-AM lettuce grown under high temperature at 35°C had damaged thylakoid organization and greater accumulation of starch granules, less chlorophyll contents and lowered net photosynthetic rate compared with those grown under 25°C. However, chlorophyll a content, chlorophyll b content, net photosynthetic rate and transpiration rate of lettuce inoculated with F. mosseae grown under high temperature increased by 4.5 %, 4.5 %, 7.7 % and 5.9 %, respectively, compared with those grown without AM fungi. Under high temperature, a depression at the P-step was observed in chlorophyll fluorescence transient in non-AM lettuce, accompanied by a significant increase in L‑step. The maximal fluorescence (Fm) of non-AM lettuce was reduced by 8.6 % as temperature increased from 25℃ to 35℃, while no significant differences were observed in Fm between AM lettuce grown under temperature at 35℃ and non-AM lettuce grown under temperature at 25℃. Absorption energy flux, dissipation energy flux and trapped energy flux per reaction center were increased with an decrease in quantum yield for electron transport (φEo) and performance index for energy conservation (PIabs) in lettuce grown under high temperature. However, opposite results were observed in lettuce inoculated with F. mosseae under high temperature. In conclusion, AM inoculaiton significantly improved lettuce tolerance to high temperature, by increasing photochemistry and preventing PSII system from heat damage.